/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Selection/MeshTopologySelectionMechanic.h
Bydefault, the Ctrl key will cause new clicks to remove from the existing selection. However, this can be changed by supplying a different function to check here.
void SetShouldRemoveFromSelectionFunc ( [TFunction](API\Runtime\Core\GenericPlatform\TFunction)< bool(void)> Func )